This invention is directed to an optically clear single phase silicone microemulsion formed with very little input of mechanical energy for mixing the components. More particularly, a ternary composition of water, a cyclic or short chain linear methyl siloxane, and a nonionic surfactant or a combination of a nonionic surfactant and a cationic surfactant, spontaneously provides optically clear one phase silicone microemulsions, when combined and gently mixed by handshaking or stirred with a magnetic stirrer.
It is well documented (U.S. Pat. No. 4,999,398) that emulsions, especially silicone emulsions, are opaque, cloudy, and tend to separate on standing. Microemulsions, in contrast, are desirable because they are thermodynamically stable and contain equilibrium microstructures that are smaller than typical emulsion droplets. Thus, the products are indefinitely stable and can be optically clear.
As used herein, the term emulsion or macroemulsion means a mixture of one immiscible liquid in another, in the form of droplets, with diameters approximately in the range of 100-1,000 nanometer (0.1-1.0 micron/1,000-10,000 angstrom .ANG.). In contrast, a microemulsion means a single or one phase transparent, thermodynamically stable, mixture of two or more immiscible liquids and one or more surfactants and co-surfactants.
In order to avoid confusion, it should be noted that the term microemulsion has been used in the literature to describe any transparent composition containing water, oil and a surfactant, including compositions which are transparent by virtue of a very small structure size and index of refraction matching. However, it is almost always apparent from the details of the preparation given which type of composition is in fact being made, considering the order of addition of the components, their polymerization, or when high energy mixing is involved.
Microemulsions are clear or transparent because they contain structures smaller than the wavelength of visible light, which is typically on order of about 500 nanometer. Furthermore, microemulsions, as that term is being used herein, contain structures that are spontaneously self-assembled aggregates consisting of oil and surfactant monolayers, or water and surfactant monolayers. Although there are distinct domains present which are composed of water and oil, the system is properly described as one phase, because the domains consist of molecular aggregates that spontaneously self-assemble.
Microemulsions may contain oil droplets dispersed in water (O/W), water droplets dispersed in oil (W/O), or they may be in the form of a bicontinuous structure or other structure. They are characterized by an ultra-low interfacial tension between the microemulsion and any excess oil-rich or water-rich phase.
A microemulsion can be recognized by several of its inherent characteristics which are that (i) it contains oil, water, and a surfactant; (ii) there is a high concentration of surfactant relative to oil; (iii) the system is optically clear; (iv) the phases do not separate by centrifugation; and (v) the system forms spontaneously.
Thus, for purposes of this invention, an emulsion is considered as containing structures having an average diameter of more than 100 nanometer (0.1 micron/1,000 angstrom .ANG.), whereas a microemulsion contains structures having an average diameter of less than 100 nanometer (0.1 micron/1,000 angstrom .ANG.), preferably less than 50 nanometer (0.05 micron/500 angstrom .ANG.). Clarity or transparency is controlled to a great extent by the structure size of the dispersed phase. The scattering of light is dependent on the structure size. Therefore, clear or transparent compositions according to this invention are a single phase without droplets or structures when viewed with the naked eye, as defined hereafter.
In addition, emulsions are recognized as inherently unstable systems separating with time. In contrast, microemulsions according to this invention form spontaneously and are stable indefinitely. The order of addition of the components does not influence their formation, and simple hand shaking in the temperature range of their stability is sufficient to cause the one phase microemulsions to form.
These spontaneously formed single phase clear microemulsions have particular value in the personal care arena. Because of the unique volatility characteristics of the cyclic methyl siloxane component of the ternary system, it can be used alone, or blended with other cosmetic fluids, to form a variety of over-the-counter personal care products.
Thus, it is useful as a carrier in antiperspirants and deodorants, since it leaves a dry feel, and does not cool the skin upon evaporation. It is lubricious and will improve the properties of skin creams, skin care lotions, moisturizers, facial treatments such as acne or wrinkle removers, personal and facial cleansers, bath oils, perfumes, colognes, sachets, sunscreens, pre-shave and after-shave lotions, shaving soaps, and shaving lathers. It can be used in hair shampoos, hair conditioners, hair sprays, mousses, permanents, depilatories, and cuticle coats, to enhance gloss and drying time, and provide conditioning benefits. In cosmetics, it will function as a leveling and spreading agent for pigments in make-ups, color cosmetics, foundations, blushes, lipsticks, eyeliners, mascaras, oil removers, color cosmetic removers, and powders. It is useful as a delivery system for oil and water soluble substances such as vitamins. When incorporated into sticks, gels, lotions, aerosols, and roll-ons, the ternary composition imparts a dry, silky-smooth, payout.
In addition, because these spontaneously formed clear one phase microemulsions exhibit a variety of advantageous and beneficial properties such as (i) clarity, (ii) very small structure size, (iii) ultra-low interfacial tensions, (iv) the ability to combine properties of water and oil in a single homogeneous fluid, (v) shelf stability, and (vi) ease of preparation; they have wide application in other arenas as well such as textile finishing.